Adiabatic equation help

[wade88]

4.132?

 

[wade88]

You "thanks" button doesnt appear to be showing Spin but Thanks mate, thats really useful.

 

[wade88]

well i was taught like this , the square root of the current squared multiplyed by the current squared multiply by the disconnection time dived by k factor

so example say 6A mcb, 0.4 disconnection time would be as follows square root of 6x6 x 6x6 x 0.4 = dived by k factor in big red book bu say we use 115 for k factor so.... now ive no calculator but you try it

result is 4.132, but i cant see a 6amp MCB requiring a 4mm CPC?!

 

[Tester Steve]

Have a look at Lennys post here.

Steve.

 

[malcolmsanford]

Use the Adiabatic Equation:

S = sqrt(I^2 x t) / k

where

S = Min CSA of CPC required in mm^2 to satisfy thermal constraints
I = Uo / Zs in Amps (In this I have just picked out a value)
t = Operating time of the protective device in seconds
k = a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material

so we have:

I = 230 / 0.56 = 410.71 A

t = 0.1 sec - from Fig 3.5 pg 250 in BRB I just used a Type C BS 60898 - the time taken will be less than this in reality - but we must use our "text box" instantaneous time if we do not have specific manufacturer data to hand.

k = 115 - from Table 54.3 for 70 C thermoplastic insulated cable

S = sqrt (410.71 ^ 2 x 0.1) / 115
S = 1.13 mm^2

Don't take note of values or anything as I just made them up but it gives you an idea on how to work the formula

 

[wade88]

Brilliant, Cheers Malc